Abstract

Correct orientation of the acetabular cup considering patient-specific functional pelvic angles is an important factor for improving outcomes and avoiding complications after total hip arthroplasty. This study introduces a new, noninvasive radiographic tool for accurately determining a patient’s specific pelvic tilt angle preoperatively, as well as accurately assessing acetabular cup orientation with respect to bony landmarks intraoperatively and postoperatively. The method was validated by imaging a bone replica model of the pelvis with implanted hip components, in comparison to digitized references, and verified with a cadaveric specimen. Pelvic tilt was measured with an accuracy of 0.1 deg and SD of 0.4 deg. Operative cup inclination and anteversion showed accuracies of 0.6 deg and 2.5 deg, with SD of 0.4 deg and 0.6 deg, respectively; these could be improved further by subtracting systematic bias. The method shows accuracy advantages over existing radiographic and fluoroscopic methods and exposes the subjects to a lower radiation dose compared to the similar computed tomography methods. These results suggest that the proposed method is feasible for assessing cup placement with reference to the functional and anatomical references. Furthermore, the ability to reference the same bony landmarks preoperatively, intraoperatively, and postoperatively has important research and clinical advantages.

Figures

Segmenting the pelvis bony landmarks on X-ray images from a cadaveric pelvis. The right and left anterior superior iliac spine (RASIS and LASIS), and right and left pubic tubercle (RPT and LPT). Because of soft tissue blocking it was more challenging to identify the bony landmarks. To help registration for each landmark, the X-ray on which the landmark was more discernible (e.g., image 2 for LASIS) was used as the primary image for registering the in-plane location, and the second image (e.g., image 1 for LASIS) was used to determine the depth of the landmark with respect to the first.

(a) Imaging setup for multiplanar radiography consists of a C-arm fluoroscope and an external motion tracking camera. Imaging the pelvis requires shifting the C-arm gantry along the longitudinal axis of the pelvis for preoperative (b) and intraoperative (c) poses of the patient [44]. The functional planes are defined from the standing pose as the vertical sagittal and frontal planes. The relationship of the functional plane to the APP can be transferred to the intraoperative (lying) situation.

Imaging process flow chart shows how the imaging and motion tracking data were used for calibration, acquiring patient images, and reconstructing the patient’s pelvis and acetabular cup orientation. The multiplanar imaging setup is based on a previously published method [34] modified to account for translations of the C-arm gantry with respect to the floor.

Biplanar radiographs of three motion tracking markers were segmented to reconstruct the 3D positions of the centers of the LED emitters of the markers (A, B, and C). This information was used to find the transformation matrix between the coordinate systems of the gantry and the camera tracking system.

Radiographic angles for capturing the bony landmarks and cup positions: (a) The first two radiographs (S1 and S2) were used to capture the right and left anterior superior iliac spine (RASIS and LASIS). (b) The C-arm was shifted distally to acquire radiographs (S3, S4, and S5) to capture the right and left pubic tubercle (RPT and LPT) landmarks and the acetabular cup.

Segmenting the pelvis bony landmarks and the acetabular cup. The numbers on the radiographs correspond with the X-ray beams in Fig. 4. The right and left anterior superior iliac spine (RASIS and LASIS), and right and left pubic tubercle (RPT and LPT), and socket shell were reconstructed each through two radiographic views. The orientation of the acetabular cup was calculated based on fitting an ellipse to the elliptical projection of the cup opening.

Definition of the anterior pelvic plane and the corresponding coordinate system (a). The orientation of the pelvis (b) was determined as tilt, rotation, and obliquity of the pelvis with respect to the functional planes.

Reconstruction of the cup orientation by analyzing the geometry of its elliptical projection. Knowing the 3D positions of the X-ray source (S) and the quadrants of the elliptical fit to the projection (Q1 to Q4 ), the orientation of the cup (n) was determined through a minimization function that placed P1 to P4 points on a circular perimeter tangent to the projection cone O-Q1 Q2 Q3 Q4 .

Digitizing the frontal pelvic plane (a), the bone landmarks (b), the cup plane (c), and the cup reference (d) geometries on a bone replica model of the pelvis implanted with a total hip arthroplasty prosthesis

Graphical output of the Joint3D software shows the calculated values of the operative anterversion (left) and inclination (right). The graphical view also overlays the digitized references to visually demonstrate the differences in the reconstruction of the individual landmarks and pelvic and acetabular cup in comparison to the reference measures.

Simulation results demonstrate the effects of uncertainties in radiographic reconstruction of the pelvic bony landmarks on the accuracies of operative anteversion (left) and inclination (right) for a range of cup orientations. These plots demonstrate that the accuracies can vary up to 2 deg for anteversion (left plot) and up to 1 deg (right plot) depending of the orientation of the acetabular cup. All measurements are in degrees.

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